Method and system for controlling fuel pressure in a gaseous fuelled internal combustion engine

ABSTRACT

A method for controlling fuel pressure in an internal combustion engine consuming a gaseous fuel and a liquid fuel comprises steps of determining a gaseous fuel pressure target value as a function of an engine operating condition, pressurizing the liquid fuel to a liquid fuel pressure based on the gaseous fuel pressure target value, and regulating gaseous fuel pressure from the liquid fuel pressure. The gaseous fuel pressure equals the gaseous fuel pressure target value to within a predetermined range of tolerance. A corresponding system controls fuel pressure in a gaseous fuelled internal combustion engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CA2013/050268 having an international filing date of Apr. 3, 2013,entitled “Method And Apparatus For Controlling Fuel Pressure In AGaseous Fuelled Internal Combustion Engine”. This application is relatedto co-pending U.S. application Ser. No. 14/504,240 which also is acontinuation of the '268 international application. The '268international application claimed priority benefits, in turn, fromCanadian Patent Application No. 2,773,651 filed on Apr. 5, 2012. The'268 international application is hereby incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a technique of controlling fuelpressure in a gaseous fuelled internal combustion engine. The techniqueinvolves determining a gaseous fuel pressure target value with respectto engine operating conditions and controlling a liquid fuel pumpingapparatus based on the target value.

BACKGROUND OF THE INVENTION

Natural gas can be used in place of diesel for supplying a Diesel-cycleengine with fuel to obtain emission and economic benefits. In theseengines it is known to use diesel as a pilot fuel since theauto-ignition temperature of natural gas is substantially greater thanthat of diesel. A small amount of diesel, normally about 5% of totalfuel introduced to the combustion chamber, is injected along withnatural gas, which is the main fuel. The diesel ignites due tocompression heat and subsequently the natural gas is ignited due to thecombustion of diesel.

A dual fuel injector separately injects two fuels into a combustionchamber of an internal combustion engine. In particular, the two fuelscan be controlled quantities of a liquid pilot fuel, such as diesel, anda gaseous fuel such as natural gas. U.S. Pat. No. 6,336,598 (the '598patent), issued Jan. 8, 2002, which is co-owned along with the presentapplication by the Applicant, discloses such a dual fuel injector thatis hydraulically actuated. The fuel injector comprises an injector body,and hydraulic fluid, liquid fuel and gaseous fuel inlet ports. Thehydraulic fluid inlet port enables pressurized hydraulic fluid to beintroduced into the interior of the injector body. A liquid seal in theinjector inhibits or prevents leakage of high-pressure gaseous fuel intothe hydraulic actuating fluid. The liquid seal is filled with thepressurized hydraulic fluid, which is substantially confined therein.The hydraulic fluid is of sufficient pressure to maintain sealing and toinhibit or prevent leakage of the gaseous fuel into the hydraulic fluid.In a preferred embodiment, the liquid pilot fuel and hydraulic fluid arethe same and both are supplied to the dual fuel injector from the dieselcommon rail. When the pilot fuel is used for sealing, the gaseous fuelis pressurized to a pressure slightly less than that of the pilot fuelpressure to inhibit or prevent leakage of gaseous fuel past a fluid sealcavity in the injector.

As disclosed in U.S. Pat. No. 6,298,833 (the '833 patent), issued Oct.9, 2001, which is also co-owned by the Applicant, it is known todynamically control sealing-fluid pressure to ensure that gaseous fuelpressure is slightly less than pilot fuel pressure for all engineoperating conditions. A pressure-balancing system, which includes apressure-balancing device such as a dome-loaded regulator, reduces thepressure differential between the sealing-fluid (the pilot fuel) and thegaseous fuel used in the dual fuel injector. At the same time, thepressure balancing system dynamically balances the sealing-fluidpressure such that the gaseous fuel pressure is equal to or slightlyless than the pressure of the pilot fuel within the injection valve. Thepressure differential between the gaseous fuel and the pilot fuel can bemaintained throughout the operating range of engine speeds, engineloads, and fuel cut-off conditions so as to inhibit or prevent leakageof compressible gaseous fuel into the pilot fuel. The reduced pressuredifferential between the gaseous fuel and the pilot fuel also reducesleakage of pilot fuel into the gaseous fuel.

Existing calibration techniques for natural gas engines employing dualfuel injectors of the type of the '598 patent and diesel as a pilot fuelfocus on the calibration of diesel rail pressures required to supportemission and fuel usage requirements across the operating range of theengine. One reason for these techniques is integration into the controlsystem of the base diesel engine which operates based on diesel railpressures. However, in systems that regulate natural gas rail pressurefrom diesel rail pressure, for example as disclosed in the '833 patent,it has been observed that the pressure differential between the dieseland natural gas rails is not consistent. Unit to unit variations andsystem aging cause the pressure differential between the pilot fuel andnatural gas to change from engine to engine and over time. This causesemission, fuel usage and engine torque targets to vary from what wereexpected. Since natural gas is the main fuel that determines emissionsand fuel usage, when diesel rail pressure is calibrated on a calibrationengine, in actual practice the diesel pressure is determined based onthe natural gas pressure that meets the emission and fuel usage targets.However, since the pressure differential between the diesel and naturalgas rails varies from engine to engine and over time then the naturalgas rail pressure will also vary from what it was on the calibrationengine. As a result engines tend to operate close to but not at the gasrail pressure within a preferred range of tolerance that meets optimumemission and fuel usage targets.

The present method and apparatus provide an improved technique forcontrolling fuel pressure in a gaseous fuelled internal combustionengine.

SUMMARY OF THE INVENTION

An improved method for controlling fuel pressure in an internalcombustion engine consuming a gaseous fuel and a liquid fuel comprisesdetermining a gaseous fuel pressure target value as a function of anengine operating condition; pressurizing the liquid fuel to a liquidfuel pressure based on the gaseous fuel pressure target value; andregulating gaseous fuel pressure from the liquid fuel pressure; suchthat the gaseous fuel pressure equals the gaseous fuel pressure targetvalue to within a predetermined range of tolerance. The gaseous fuel canbe natural gas, or can be selected from the group consisting of methane,propane, butane, ethane and hydrogen. The liquid fuel can be diesel. Theengine operating condition comprises at least one of engine speed,engine torque and base engine fuelling quantity. The gaseous fuelpressure target value is calibrated on a calibration engine to optimizeat least one engine parameter, which can be emissions, fuel usage andengine torque, as well as other common engine parameters.

In one aspect, the method further comprises measuring the gaseous fuelpressure; and pressurizing the liquid fuel such that the measuredgaseous fuel pressure equals the gaseous fuel pressure target value towithin a predetermined range of tolerance.

In another aspect, the method further comprises calculating a liquidfuel pressure target value as a function of the gaseous fuel pressuretarget value and a nominal pressure differential between the liquid fuelpressure and the gaseous fuel pressure; and pressurizing the liquid fuelsuch that the liquid fuel pressure equals the liquid fuel pressuretarget value to within a predetermined range of tolerance. The liquidfuel pressure target value can be stored in a table indexed by theparameters representing the engine operating condition.

In yet another aspect, the method further comprises determining anactual pressure differential between the liquid fuel pressure and thegaseous fuel pressure; and employing the actual pressure differentialinstead of the nominal pressure differential when calculating the liquidfuel pressure target value. The actual pressure differential can bestored, for example in a memory of an engine controller; and the storedactual pressure differential can be employed instead of the nominalpressure differential when calculating the liquid fuel pressure targetvalue. The gaseous fuel at the gaseous fuel pressure and the liquid fuelat the liquid fuel pressure can be delivered to an injection valve, andthe liquid fuel can form a fluid seal for the gaseous fuel within theinjection valve. The gaseous fuel pressure target value can be one of aplurality of gaseous fuel pressure target values, and each gaseous fuelpressure target value can be associated with a respective engineoperating condition through at least one of a mathematical functionparameterized by engine operating conditions and a table indexed byengine operating conditions. The gaseous fuel pressure target value canbe determined by interpolating between at least two gaseous fuelpressure target values in the table.

In yet again another aspect, the method further comprises variablyregulating the gaseous fuel pressure independently from the liquid fuelpressure; and controlling a pressure differential between the liquidfuel pressure and the gaseous fuel pressure, for example based on engineoperating conditions.

In still yet again another aspect, the step of determining the gaseousfuel pressure target value comprises determining a calibrated liquidfuel pressure target value as a function of the engine operatingcondition, the gaseous fuel pressure target value is equal to adifference between the calibrated liquid fuel pressure target value anda calibrated pressure differential between a liquid fuel calibrationpressure and a gaseous fuel calibration pressure to within apredetermined range of tolerance, and in this aspect the method furthercomprises determining an actual pressure differential between the liquidfuel pressure and the gaseous fuel pressure; calculating an actualliquid fuel pressure target value as a function of the calibrated liquidfuel pressure target value, the calibrated pressure differential and theactual pressure differential, the gaseous fuel pressure target value isequal to a difference between the actual liquid fuel pressure targetvalue and the actual pressure differential to within a predeterminedrange of tolerance; and pressurizing the liquid fuel such that theliquid fuel pressure equals the actual liquid fuel pressure target valueto within a predetermined range of tolerance. The actual pressuredifferential again can be stored in the memory of the engine controller.

In a further aspect the method comprises determining an actual pressuredifferential between the liquid fuel pressure and the gaseous fuelpressure; and comparing the actual pressure differential with at leastone of a nominal pressure differential and a previously determinedactual pressure differential and rejecting the actual pressuredifferential if it is more than at least one of a predeterminedpercentage and fixed amount from the nominal pressure differential andthe previously determined actual pressure differential.

In still a further aspect the method comprises determining an actualpressure differential between the liquid fuel pressure and the gaseousfuel pressure; comparing the actual pressure differential with a nominalpressure differential; and determining from the comparison at least oneof an aging characteristic and a health of at least one of an injectionvalve, a fuel system of the internal combustion engine and a pressureregulator for regulating the gaseous fuel pressure from the liquid fuelpressure. Similarly, when the actual pressure differential is a firstmeasured pressure differential, the method further comprises comparing asecond measured pressure differential with the first measured pressuredifferential; and determining from the comparison at least one of anaging characteristic and a health of at least one of an injection valve,a fuel system of the internal combustion engine and a pressure regulatorfor regulating the gaseous fuel pressure from the liquid fuel pressure.

Again, in another aspect the method further comprises determining actualpressure differentials between the liquid fuel pressure and the gaseousfuel pressure as a function of at least one engine operating conditionat multiple points in time; and storing the actual pressuredifferentials. The stored actual pressure differentials from at leastone and preferably more than one internal combustion engine can beanalyzed to determine at least one of a normal characteristic and afailure characteristic for a pressure regulator. The stored actualpressure differentials can be compared to the failure characteristic ofthe pressure regulator; and an operator can be warned or the liquid fuelpressure can be reduced or limited when at least a portion of the storedactual pressure differentials matches the failure characteristic towithin a predetermined range of tolerance.

An improved apparatus for controlling fuel pressure in an internalcombustion engine consuming a gaseous fuel and a liquid fuel comprises aliquid fuel pumping apparatus for pressurizing the liquid fuel from asupply of liquid fuel to a liquid fuel pressure in a liquid fuel rail; apressure regulator associated with a gaseous fuel line operative toregulate the gaseous fuel from a supply of gaseous fuel to the gaseousfuel line at a gaseous fuel pressure; and an electronic controllerprogrammed to determine a gaseous fuel pressure target value as afunction of an engine operating condition; and command the liquid fuelpumping apparatus to pressurize the liquid fuel as a function of thegaseous fuel pressure target value; such that the gaseous fuel pressureequals the gaseous fuel pressure target value to within a predeterminedrange of tolerance. The liquid fuel pumping apparatus can comprise atleast one of a liquid fuel pump and an inlet metering valve connectedbetween the supply of liquid fuel and the liquid fuel pump. The pressureregulator can be a dome-loaded regulator.

In one aspect the apparatus further comprises a gaseous fuel pressuresensor for measuring pressure within the gaseous fuel line, theelectronic controller receives signals from the gaseous fuel pressuresensor representative of measured gaseous fuel pressure; the electroniccontroller is further programmed to regulate liquid fuel flow from theliquid fuel pumping apparatus to reduce differences between the measuredgaseous fuel pressure and the gaseous fuel pressure target value.

In another aspect the apparatus further comprises a liquid fuel pressuresensor for measuring pressure within the liquid fuel rail, theelectronic controller receives signals from the liquid fuel pressuresensor representative of measured liquid fuel pressure; the electroniccontroller further programmed to calculate a liquid fuel pressure targetvalue as a function of the gaseous fuel pressure target value and anominal pressure differential between the liquid fuel pressure and thegaseous fuel pressure; and regulate liquid fuel flow from the liquidfuel pumping apparatus to reduce differences between the measured liquidfuel pressure and the liquid fuel pressure target value. In this aspectthe apparatus can further comprise a gaseous fuel pressure sensor formeasuring pressure within the gaseous fuel line, the electroniccontroller receives signals from the gaseous fuel pressure sensorrepresentative of measured gaseous fuel pressure; and the electroniccontroller is further programmed to calculate an actual pressuredifferential between the liquid fuel pressure and the gaseous fuelpressure by subtracting the measured gaseous fuel pressure from themeasured liquid fuel pressure; and employ the actual pressuredifferential instead of the nominal pressure differential whencalculating the liquid fuel pressure target value.

In yet another aspect, the pressure regulator is a variable pressureregulator and the electronic controller commands the variable pressureregulator to regulate the gaseous fuel pressure thereby controlling apressure differential between the liquid fuel pressure and the gaseousfuel pressure. The electronic controller can be programmed to adjust thepressure differential based on engine operating conditions.

In yet again another aspect the apparatus further comprises a gaseousfuel pressure sensor for measuring pressure within the gaseous fuelline, the electronic controller receives signals from the gaseous fuelpressure sensor representative of measured gaseous fuel pressure; aliquid fuel pressure sensor for measuring pressure within the liquidfuel rail, the electronic controller receives signals from the liquidfuel pressure sensor representative of the measured liquid fuelpressure; and the electronic controller is further programmed todetermine the gaseous fuel pressure target value by determining acalibrated liquid fuel pressure target value as a function of the engineoperating condition, the gaseous fuel pressure target value is equal toa difference between the calibrated liquid fuel pressure target valueand a calibrated pressure differential between a liquid fuel calibrationpressure and a gaseous fuel calibration pressure to within apredetermined range of tolerance; calculate an actual pressuredifferential between the liquid fuel pressure and the gaseous fuelpressure by subtracting the measured gaseous fuel pressure from themeasured liquid fuel pressure; calculate an actual liquid fuel pressuretarget value as a function of the calibrated liquid fuel pressure targetvalue, the calibrated pressure differential and the actual pressuredifferential, the gaseous fuel pressure target value is equal to adifference between the actual liquid fuel pressure target value and theactual pressure differential to within a predetermined range oftolerance; and regulate liquid fuel flow from the liquid fuel pumpingapparatus to reduce differences between the measured liquid fuelpressure and the actual liquid fuel pressure target value.

In a further aspect, the apparatus further comprises a gaseous fuelpressure sensor for measuring pressure within the gaseous fuel line, theelectronic controller receives signals from the gaseous fuel pressuresensor representative of measured gaseous fuel pressure; a liquid fuelpressure sensor for measuring pressure within the liquid fuel rail, theelectronic controller receives signals from the liquid fuel pressuresensor representative of measured liquid fuel pressure; and theelectronic controller is further programmed to calculate actual pressuredifferentials between the liquid fuel pressure and the gaseous fuelpressure as a function of at least one engine operating condition atmultiple points in time, the actual pressure differentials arecalculated by subtracting the measured gaseous fuel pressure from themeasured liquid fuel pressure; and store the actual pressuredifferentials. The electronic controller can be further programmed tostore at least one of a normal characteristic and a failurecharacteristic for the pressure regulator; compare the stored actualpressure differentials to the failure characteristic; and warn anoperator or limit the liquid fuel pressure when at least a portion ofthe stored actual pressure differentials resemble the failurecharacteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial schematic view of a fuel system for an internalcombustion engine according to one embodiment of the present method andapparatus for controlling fuel pressure in a gaseous fuelled internalcombustion engine.

FIG. 2 is a chart illustrating fuel pressure and bias versus enginetorque for the internal combustion engine of FIG. 1 operating at oneengine speed.

FIG. 3 is a chart illustrating engine torque versus engine speed for theinternal combustion engine of FIG. 1.

FIG. 4 is a chart illustrating fuel pressure versus time for the fuelsystem of FIG. 1.

FIG. 5 is flow chart for a fuel pressure control algorithm for the fuelsystem of FIG. 1 according to a first embodiment.

FIG. 6 is flow chart for a fuel pressure control algorithm for the fuelsystem of FIG. 1 according to a second embodiment.

FIG. 7 is flow chart for a fuel pressure control algorithm for the fuelsystem of FIG. 1 according to a third embodiment.

FIG. 8 is a flow chart for a fuel pressure control algorithm for thefuel system of FIG. 1 according to a fourth embodiment.

FIG. 9 a partial schematic view of a fuel system for an internalcombustion engine according to a second embodiment of the present methodand apparatus for controlling fuel pressure in a gaseous fuelledinternal combustion engine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

Referring to FIG. 1, there is shown a simplified view of fuel system 10for supplying a liquid fuel and a gaseous fuel at injection pressure toinjection valve 20 in an internal combustion engine (not shown).Injection valve 20 is a dual-fuel injector that introduces the liquidfuel and the gaseous fuel separately and independently directly into acombustion chamber (not shown) of the internal combustion engine. Theliquid fuel acts as both a pilot fuel for igniting the fuel mixtureinside the combustion chamber and a sealing fluid for sealing thegaseous fuel inside injection valve 20. In the present example theliquid fuel is diesel fuel, but can be other types of liquid fuel thatare suitable for compression ignition inside the combustion chamber. Thegaseous fuel is the main fuel for combustion in the engine. In thepresent example the gaseous fuel is natural gas, but can be other typesof gaseous fuels which benefit from the ignition assist provided by aliquid fuel that is more easily ignitable from compression heat.

Fuel system 10 comprises a liquid fuel supply 30 and a gaseous fuelsupply 40. Liquid fuel supply 30 can be a liquid fuel tank, whichsupplies the liquid fuel through line 50 to liquid fuel pumpingapparatus 60. Gaseous fuel supply 40 is an accumulator in the presentembodiment, but in other embodiments supply 40 can be a gas cylinderholding compressed natural gas (CNG). In the present example supply 40accumulates gaseous fuel from upstream supply line 70, which can be acommercial or residential gas line, or a feed pipe from a supply ofliquefied gaseous fuel such as liquefied natural gas (LNG) or liquefiedpetroleum gas (LPG). In other embodiments a compressor may be requiredto elevate the pressure of the gaseous fuel above that required forinjection by valve 20 into the combustion chamber in the internalcombustion engine. Supply 40 provides gaseous fuel to pressure regulator90 through line 80.

Liquid fuel pumping apparatus 60 pressurizes the liquid fuel to apressure suitable for injection by injection valve 20 into thecombustion chamber. Pumping apparatus 60 comprises a liquid fuel pump inthe present embodiment. In other embodiments pumping apparatus 60 canfurther comprise an inlet metering valve between supply 30 and theliquid fuel pump. Pressurized liquid fuel is delivered to injectionvalve 20 through liquid fuel rail 100. Although only one injection valveis shown in FIG. 1, it is understood that in most embodiments thisinjection valve is one of a plurality of injection valves, eachassociated with a respective combustion chamber, and in such embodimentsliquid fuel rail 100 is what is known as a command rail that deliversliquid fuel to all of the injection valves. Pressurized liquid fuel isalso delivered to pressure regulator 90 through line 110. Rail 100 andline 110 both receive liquid fuel from pumping apparatus 60. In thepresent embodiment rail 100 and line 110 are connected to pumpingapparatus 60 through a common line, although this is not a requirement.The liquid fuel pressure in rail 100 is equal to the liquid fuelpressure in line 110 to within a predetermined range of tolerance, andpreferably these pressures are equal.

Pressure regulator 90 is responsive to liquid fuel pressure in line 110to regulate gaseous fuel pressure in line 120 below liquid fuel pressurein line 110 and rail 100 by a predetermined amount within a range oftolerance. The operation of regulator 90 is described in further detailin the aforementioned '833 patent. In the present example regulator 90is a dome-loaded regulator, which is well understood by those familiarwith this technology. Injection valve 20 receives gaseous fuel from line120, and in embodiments with more than one valve 20 all such injectionvalves receive gaseous fuel from line 120.

With reference to FIG. 2, the differential pressure between liquid fuelpressure (LFP) in rail 100 and gaseous fuel pressure (GFP) in line 120is a system characteristic, also called bias and is substantiallyconstant across the operating range of the engine. As seen in FIG. 2,when the liquid and gaseous fuel pressures vary across the range ofengine torques the bias remains substantially constant. Thisrelationship is similar across the range of engine speeds. However, thebias can vary due to a number of factors. For example, unit to unitvariations in pressure regulator 90 results in different levels of biasfrom engine to engine.

As the internal combustion engine is operated, the bias can change dueto aging of the system. For example, changes to operatingcharacteristics of components in pressure regulator 90 can lead to newor increased internal leakage which contributes to bias drift. Other newor increased leakage that develops in fuel system 10, injection valve 20and the internal combustion engine further contributes to bias drift. Asthe liquid fuel pressure is varied across the range of pressuresrequired for operating the internal combustion engine the bias can alsodrift. In engines where gaseous fuel is the main fuel and the liquidfuel is the pilot fuel it has been learned that, due to the variabilityof the bias, calibrating gaseous fuel pressure in line 120 for the rangeof engine operating conditions, instead of calibrating liquid fuelpressure in rail 100, results in emission, torque and/or fuel usageimprovements. Improvements in fuel usage results in improvements to fueleconomy and fuel consumption. This is in contrast to the industrypractice for common rail diesel-cycle engines of calibrating diesel railpressure (liquid fuel pressure) in rail 100 based on engine operatingconditions and then deriving gaseous fuel pressure from the liquid fuelpressure.

In a new calibration technique for common rail diesel-cycle enginesemploying a liquid pilot fuel and a gaseous main fuel, gaseous fuelpressure target values for line 120 are calibrated for the range ofengine operating conditions. In the present example the engine operatingconditions comprise the range of engine speeds and engine torquesrequired for operating the engine. However, this is illustrative onlyand the engine operating conditions can be other measured and determinedengine parameters. For example, the diesel fuelling quantity which thebase diesel engine would inject into the combustion chamber if theengine was operated only on diesel can be employed as an engineoperating condition for which to select the gaseous fuel pressure targetvalues.

With reference to FIG. 3, an engine torque-speed chart is shown. Line160 is a limit line above which the engine should not be operated, andthe area under line 160 represents the safe operating region for theengine. As part of the new calibration technique this area is subdividedinto sub-regions represented by the dotted lines in FIG. 3. As isunderstood by those familiar with this technology, the number, size andshape of sub-regions can vary, and those shown in FIG. 3 are forillustrative purposes only. For each sub-region a predetermined enginespeed and engine torque value are selected, for example as representedby calibration coordinate (SPD1, TR1), and the gaseous fuel pressuretarget value is determined for this coordinate such that emissionsand/or fuel usage are optimized. The gaseous fuel pressure target valuecan also be selected to match actual torque with demanded torque.

All the calibration coordinates and target values are tabulated andstored in a calibration table in controller 150. In other embodiments itis possible to perform curve fitting techniques on the calibrationcoordinates and target values to determine a mathematical function (aformula) which can be employed to calculate the gaseous fuel pressuretarget value as a function of engine operating conditions. Other engineoperating conditions can be employed to determined calibrated gaseousfuel pressure target values. For example, gaseous fuel pressure targetvalues can be calibrated as a function of gas fuelling versus enginespeed or liquid fuelling versus engine speed.

Referring back to FIG. 1, fuel system 10 further comprises electroniccontroller 150 and optional pressure sensors 130 and 140. Electroniccontroller 150 can comprise both hardware and software components. Thehardware components can comprise digital and/or analog components. Inthe present example electronic controller 150 is a computer comprising aprocessor and memories, including one or more permanent memories, suchas FLASH, EEPROM and a hard disk, and a temporary memory, such as SRAMor DRAM, for storing and executing a program. In another preferredembodiment electronic controller 150 is an engine control unit (ECU) forthe engine. As used herein, controller 150 is also referred to as ‘thecontroller’. Pressure sensor 130 measures liquid fuel pressure in commonrail 100, and pressure sensor 140 measures gaseous fuel pressure in line120. Electronic controller 150 is responsive to signals received frompressure sensors 130 and 140 that are representative of their respectivepressures to command pumping apparatus 60 to pressurize the liquid fuelaccordingly, as will be described in further detail below. At least oneof pressure sensors 130 and 140 are required when operating thetechniques disclosed herein.

With reference to FIG. 4, the gaseous fuel pressure (GFP) and the liquidfuel pressure (LFP) are preferably measured when the pressures in rail100 and line 120 have stabilized, for example at time T1. Transientpressure conditions that are present in lines 100, 110 and 120 when theliquid fuel pressure is being changed from one pressure to another cancreate noise that adversely affects readings from pressure sensors 130and 140. In addition, the liquid fuel pressure can change at a differentrate than the gaseous fuel pressure during transients, which can resultin inaccurate bias readings. It is possible that the pressure noise canbe filtered using analog and/or digital techniques. The controllerpreferably uses pressure measurements that are obtained at a stablepoint such as at time T1, for example when the engine speed and torquehave stabilized.

Referring now to FIG. 5, a technique for controlling fuel pressure infuel system 10 will now be described according to a first embodiment.The flow chart of FIG. 5 illustrates a control algorithm that isperformed by electronic controller 150. The starting point for thealgorithm is in step S200 and which is a change in engine operatingconditions that requires a change in gaseous fuel pressure in line 120and liquid fuel pressure in rail 100. In step S210 electronic controller150 determines a gaseous fuel pressure target value as a function ofengine operating conditions. For example, in a preferred embodiment theengine operating conditions employed by controller 150 to determine thetarget value can be engine speed and engine torque. During step S210controller 150 determines the gaseous fuel pressure target value for thecurrent engine operating condition by employing a mathematical functionto interpolate between the calibrated values in the calibration table.When the current engine operating condition comprises substantiallyequivalent engine parameter values employed during calibration,controller 150 can perform a look-up function directly to retrieve thetarget value from the calibration table.

As previously mentioned, in other embodiments the mathematical functioncan be employed to determine the gaseous fuel pressure target value. Instep S220 the controller commands the liquid fuel pumping apparatus 60to pump liquid fuel from supply 30 to pressurize the liquid fuel in rail100 and line 110 such that gaseous fuel pressure in line 120 equals thegaseous fuel pressure target value to within a predetermined range oftolerance. The controller monitors the signal received from gaseous fuelpressure sensor 140 to regulate the flow of liquid fuel from pumpingapparatus 60 such that the measured gaseous fuel pressure equals thetarget value. The liquid fuel pressure sensor 130 is not required, butcan be used as a safety device for monitoring pressure in rail 100 andline 110 such that the controller can warn an operator or takecorrective actions in the event of abnormal liquid fuel pressure. Bythis method the engine operates at the calibrated gaseous fuel pressureswhich is the opposite of how prior art engines have been operated whenfuelled with directly injected gaseous fuel and liquid fuel wheregaseous fuel pressure is a function of a commanded target liquid fuelpressure.

Referring now to FIG. 6 a second embodiment of the disclosed techniquefor controlling fuel pressure in fuel system 10 will now be described.This embodiment is similar to the previous embodiment of FIG. 5, andlike parts will not be described in detail. The starting point for thealgorithm in step S300 is a change in engine operating conditions thatrequires a change in gaseous fuel pressure in line 120 and liquid fuelpressure in rail 100. In step S310 the controller determines a gaseousfuel pressure target value as a function of engine operating conditions,for example engine speed and engine torque. In step 320 the controllercalculates a liquid fuel pressure target value as a function of thegaseous fuel pressure target value and a nominal system characteristic.The nominal system characteristic is a nominal pressure differential(the bias) between liquid fuel pressure in rail 100 and gaseous fuelpressure in line 120 maintained by pressure regulator 90. The nominalpressure differential is an expected value determined empirically orfrom manufacturer specifications, and is accurate to within apredetermined range of tolerance. Preferably the nominal pressuredifferential is determined empirically for each pressure regulator 90such that variations in bias from unit to unit can be taken intoaccount. The liquid fuel pressure target value is a function of thegaseous fuel target value calculated in step S320 by adding the gaseousfuel pressure target value and the nominal pressure differential.

In other embodiments the liquid fuel pressure target value for allengine operating points (the coordinates within the safe operatingregion shown in FIG. 3) can be calculated beforehand after calibrationof the gaseous fuel pressure target values for each engine and itsrespective pressure regulator 90, and in step S320 the controllerretrieves the liquid fuel pressure target value from a look-up tablestored in memory accessible by the controller. That is the steps of S310and S320 can be replaced by a step of looking up the liquid fuelpressure target value based on the engine operating conditions. In stepS330 the controller commands the liquid fuel pumping apparatus 60 topump liquid fuel from supply 30 to pressurize the liquid fuel in rail100 and line 110 such that liquid fuel pressure in rail 100 equals theliquid fuel pressure target value to within a predetermined range oftolerance. By determining the liquid fuel pressure target value as afunction of the gaseous fuel pressure target value and the nominalpressure differential for regulator 90, the controller is commandingpumping apparatus 60 based on the gaseous fuel pressure target value.The controller monitors the signal received from liquid fuel pressuresensor 130 to regulate the flow of liquid fuel from pumping apparatus 60such that measured liquid fuel pressure equals the liquid fuel pressuretarget value to within a predetermined range of tolerance. The gaseousfuel pressure sensor 140 is not required, but can be used as a safetydevice for monitoring pressure in line 120 such that the controller canwarn an operator or take corrective actions in the event of abnormalgaseous fuel pressure.

Referring now to FIG. 7 a third embodiment of the disclosed techniquefor controlling fuel pressure in fuel system 10 will now be described.This embodiment is similar to the previous embodiment of FIG. 6, andlike parts will not be described in detail if at all. Steps S400, S410and S430 are the same as previous steps S300, S310 and S330 respectivelyand are not discussed. After determining the gaseous fuel pressuretarget value as a function of engine operating conditions in step S410,the controller calculates the liquid fuel pressure target value as afunction of the gaseous fuel pressure target value and an actualpressure differential in step S420. In the previous embodiment of FIG. 6unit to unit variation in bias for pressure regulator 90 was accountedfor by empirically determining the nominal pressure differential foreach pressure regulator 90. However, as the engine is operated and thesystem ages the actual pressure differential can vary from the nominalpressure differential.

The actual pressure differential is determined in steps S450 throughS470. In step S450 the controller measures the liquid fuel pressure inrail 100 and the gaseous fuel pressure in line 120 by receivingcorresponding signals representative of these pressures from sensors 130and 140. The actual pressure differential is calculated in step S460 bysubtracting the measured gaseous fuel pressure from the measured liquidfuel pressure. In this step the controller can reject the measuredactual liquid fuel pressure differential if it is more than apredetermined percentage or fixed amount from the nominal pressuredifferential or a previously measured actual pressure differential suchthat erroneous readings can be filtered out. For example, it is expectedthat the actual pressure differential does not vary greatly in valuefrom the nominal pressure differential, and any measured actual pressuredifferential can be discarded if it is uncharacteristically different invalue from the nominal pressure differential or a previously measuredactual pressure differential which could be indicative of noise which ispresent during a transient condition. It is important to emphasize thateven modest changes in the actual pressure differential value canadversely influence emission and fuel usage targets for the engine ifthis change is not accounted for while pressurizing rail 120. In stepS470 the controller stores the actual pressure differential in a memoryalong with the corresponding engine operating conditions. Other metadataassociated with the actual pressure differential and/or the engineoperating conditions can also be stored. For example, a timestamp ofwhen the actual pressure differential was calculated can be stored alongwith other engine parameters. In a preferred embodiment the controllerstores the actual pressure differential at least each time its currentvalue changes. Preferably, when storing a new actual pressuredifferential the controller does not overwrite the previously storedvalue such that a history of actual pressure differentials can beobtained. The controller employs the current actual pressuredifferential when calculating the liquid fuel pressure target value instep S420.

The history of stored actual pressure differentials (bias history) fromone or more internal combustion engines can be analyzed to determine anormal characteristic and a failure characteristic for the bias. Thesecharacteristics can be used to determine whether pressure regulator 90is operating within manufacturer's specifications or not, and can beused to proactively recognize or predict when the regulator needs to beserviced or replaced before it fails. The bias histories can be obtainedfrom engines operating in a controlled test cell environment or can beobtained from engines operating when in service in the field. In thetest cell environment accelerated testing can be done until pressureregulator 90 fails, or known faulty or adapted pressure regulators 90can be used to obtain bias failure data. The bias histories can also beobtained from engines in the field while they are being serviced or whendeployed by telemetry.

Referring now to FIG. 8 a fourth embodiment of the disclosed techniquefor controlling fuel pressure in fuel system 10 will now be described.This embodiment is similar to the previous embodiment of FIG. 7, andlike parts will not be described in detail if at all. Steps S500, S550,S560 and S570 are the same as previous steps S400, S450, S460 and S470respectively and are not discussed. As in the previous embodiments, foreach engine operating condition the engine is operated at respectivegaseous fuel pressure target values to within a predetermined range oftolerance. In the present embodiment the manner by which the engineoperates at a particular gaseous fuel pressure target value is by way ofadjusting a calibrated liquid fuel pressure target value based on adifference between an actual pressure differential and a calibratedpressure differential. In step S510, in response to a change in engineoperating conditions, the controller determines the calibrated liquidfuel pressure target value as a function of engine operating conditions,for example engine speed and engine torque. As illustrated in eqn. 1,the calibrated liquid fuel pressure target value (LFPTV_(C)) is equal tothe gaseous fuel pressure target value (GFPTV), as calibrated on thecalibration engine for the engine operating conditions, plus thecalibrated pressure differential (PD_(C)) between the liquid fuelpressure and the gaseous fuel pressure on the calibration engine. In thepresent embodiment, as in previous embodiments, the objective is tooperate at the gaseous fuel pressure target value for respective engineoperating conditions, which is different from prior art techniques whichoperate at a liquid fuel pressure target value without any correctionfor the actual pressure differential which results in drifting away fromoperating at the gaseous fuel pressure target value.LFPTV_(C)=GFPTV+PD_(C)  eqn.1

For the current embodiment a new calibration technique is employed thatis similar to the existing calibration techniques discussed previouslyin the background and includes a new step, which is described below,that is different from the existing techniques. As is already performedin the existing techniques, during calibration the calibrated liquidfuel pressure target values that optimize engine parameters such as atleast one of emissions and fuel usage are recorded for the range ofengine operating conditions. Different from existing techniques, the newstep comprises recording the calibrated pressure differential betweenthe liquid fuel pressure and the gaseous fuel pressure on thecalibration engine. The calibrated liquid fuel pressure target valuesand the calibrated pressure differential together define the gaseousfuel pressure target values that provided optimum operating parametersfor the range of engine operating conditions on the calibration engine.A table or mathematical function that defines the calibrated liquid fuelpressure target values as a function of engine operating conditions iscompiled and stored in controller 150 along with the calibrated pressuredifferential which can also be defined as a function of engine operatingconditions. For example, the calibrated pressure differential on thecalibration engine can be measured before, during and after calibrationto determine whether any deviation in its value has occurred, or theactual pressure differential on the calibration engine can be determinedand recorded for each engine operating condition for which calibrationis performed. The calibrated pressure differential on the calibrationengine should not vary significantly before, during and aftercalibration.

Returning to step S510, by determining the calibrated liquid fuelpressure target value and having knowledge of the calibrated pressuredifferential on the calibration engine, in effect the gaseous fuelpressure target value that optimized an engine parameter on thecalibration engine for the current engine operating condition is known.However, the actual pressure differential changes from engine to engine,and in these other engines knowing the calibrated liquid fuel pressuretarget value and the calibrated pressure differential does not provideenough information to operate these engines at the gaseous fuel pressuretarget value to within a predetermined range of tolerance. In step S520the controller calculates an actual liquid fuel pressure target value asa function of the calibrated liquid fuel pressure target value, thecalibrated pressure differential on the calibration engine and theactual pressure differential on the present engine received from stepS570. As illustrated in eqn. 2, the actual liquid fuel pressure targetvalue (LFPTV_(A)) is equal to the calibrated liquid fuel pressure targetvalue (LFPTV_(C)) plus the difference between the actual pressuredifferential (PD_(A)) on the present engine and the calibrated pressuredifferential (PD_(C)) on the calibration engine.LFPTV_(A)=LFPTV_(C)+(PD_(A)−PD_(C))  eqn.2

The actual liquid fuel pressure target value represents what thepressure in rail 100 should be such that line 120 operates at thegaseous fuel pressure target value as determined on the calibrationengine for the current engine operating condition. The actual liquidfuel pressure target value takes the actual bias into consideration onthe present engine. Eqn. 3 illustrates that the gaseous fuel pressuretarget value is equal to the actual liquid fuel pressure target valueminus the actual pressure differential, which is obtained bysubstituting eqn. 1 into eqn. 2 and solving for GFPTV.GFPTV=LFPTV_(A)−PD_(A)  eqn.3

In step S530 the controller commands the liquid fuel pumping apparatus60 to pump liquid fuel from supply 30 to pressurize the liquid fuel inrail 100 and line 110 such that liquid fuel pressure in rail 100 equalsthe actual liquid fuel pressure target value to within a predeterminedrange of tolerance. By determining the actual liquid fuel pressuretarget value as a function of the calibrated liquid fuel pressure targetvalue, the calibrated pressure differential on the calibration engineand the actual pressure differential on the present engine, thecontroller is commanding pumping apparatus 60 based on the gaseous fuelpressure target value that meets the emission and/or fuel usage targets.The controller monitors the signal received from liquid fuel pressuresensor 130 to regulate the flow of liquid fuel from pumping apparatus 60such that measured liquid fuel pressure equals the actual liquid fuelpressure target value to within a predetermined range of tolerance.

Referring now to FIG. 9, there is shown a simplified view of anotherembodiment of fuel system 10 for supplying liquid fuel and gaseous fuelat injection pressure to injection valve 20. This embodiment is similarto the embodiment of FIG. 1 and only the differences are discussed.Pressure regulator 90 is a variable pressure regulator that iscontrollable by controller 150 to adjustably regulate the gaseous fuelpressure in line 120. In this embodiment regulator 90 is not directlyresponsive to liquid fuel pressure. The controller monitors the pressuresignals from sensors 130 and 140 and commands pumping apparatus 60 andpressure regulator 90 such that a target pressure differential betweenrail 100 and line 120 is maintained to within a predetermined range oftolerance. In existing techniques, which employ a dome loaded regulatorfor pressure regulator 90, the pressure differential between the liquidfuel and the gaseous fuel is selected by the minimum required to ensureno gaseous fuel leaks to a liquid fuel drain line (not shown) to withina predetermined range of tolerance, and is set by the worst casecondition such as a high fuel flow operating point like peak power. Thatis, the pressure differential between the liquid fuel and the gaseousfuel is selected to reduce and preferably minimize leakage of gaseousfuel to the liquid fuel drain line that returns liquid fuel frominjection valve 20 to supply 30.

Excessively increasing the pressure differential causes liquid fuel toleak into gaseous fuel and can detrimentally change needle motion ininjection valve 20. It is preferable to reduce the pressure differentialduring lower load operating points other than peak power such that bothgaseous fuel leak to drain is reduced and preferably minimized andliquid fuel leak into gaseous fuel is reduced and preferably minimized.That is, an optimum pressure differential is preferred at each engineoperating condition. Controller 150 can adjust the pressure differentialas a function of engine operating conditions to optimize the performanceof the fluid seal in injection valve 20 by reducing bias at idle andlower load conditions and progressively increasing bias at higher loadconditions. When pressure regulator 90 is a dome loaded regulator, as inexisting fuel systems discussed in the previously discussed '833 patent,the liquid fuel pressure can change at a different rate than the gaseousfuel pressure during transients. The present embodiment has theadvantage of independently controlling the liquid fuel pressure from thegaseous fuel pressure such that a desired pressure differential, toreduce and preferably minimize fuel leakage in injection valve 20, canbe maintained during transients.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, that theinvention is not limited thereto since modifications can be made bythose skilled in the art without departing from the scope of the presentdisclosure, particularly in light of the foregoing teachings.

What is claimed is:
 1. A fuel system for an injection valve thatintroduces a liquid fuel and a gaseous fuel separately and independentlyof each other into a combustion chamber of an internal combustionengine, the fuel system comprising: a liquid fuel supply; a gaseous fuelsupply; a liquid fuel pumping apparatus in fluid communication with theliquid fuel supply and supplying the liquid fuel to the injection valveat a pressure suitable for injecting the liquid fuel into the combustionchamber; a pressure regulator in fluid communication with the gaseousfuel supply and supplying the gaseous fuel to the injection valve at apressure suitable for injecting the gaseous fuel into the combustionchamber, the pressure regulator being unresponsive to liquid fuelpressure downstream from the liquid fuel pumping apparatus; a firstpressure sensor for measuring liquid fuel pressure downstream from theliquid fuel pumping apparatus; a second pressure sensor for measuringgaseous fuel pressure downstream from the pressure regulator; and acontroller operatively connected with the liquid fuel pumping apparatus,the pressure regulator, the first pressure sensor and the secondpressure sensor, and programmed to: monitor pressure signals from thefirst pressure sensor and the second pressure sensor that arerepresentative of liquid fuel pressure and gaseous fuel pressurerespectively; and command the liquid fuel pumping apparatus and thepressure regulator such that a target pressure differential between theliquid fuel pressure and the gaseous fuel pressure is maintained withina predetermined range of tolerance.
 2. The fuel system of claim 1,wherein the pressure regulator is a variable pressure regulator that canadjustably regulate the gaseous fuel pressure.
 3. The fuel system ofclaim 1, wherein the target pressure differential is a function ofengine operating conditions.
 4. The fuel system of claim 3, wherein thetarget pressure differential increases as engine load increases.
 5. Thefuel system of claim 3, wherein the target pressure differential isreduced at lower load engine operating conditions compared to higherload engine operating conditions.
 6. The fuel system of claim 5, whereinthe target pressure differential is reduced at idle compared to higherload engine operating conditions.
 7. The fuel system of claim 1, whereinthe target pressure differential is maintained during transient engineoperating conditions.
 8. A method for controlling liquid fuel pressureand gaseous fuel pressure for an injection valve that introduces aliquid fuel and a gaseous fuel separately and independently of eachother into a combustion chamber of an internal combustion engine, themethod comprising: monitoring the liquid fuel pressure; monitoring thegaseous fuel pressure; commanding a pump to pressurize the liquid fuel;and commanding a regulator to regulate the gaseous fuel; wherein theregulator is unresponsive to the liquid fuel pressure downstream fromthe pump and a target pressure differential in response to themonitoring is maintained between the liquid fuel pressure and thegaseous fuel pressure within a predetermined range of tolerance.
 9. Themethod of claim 8, wherein the target pressure differential is afunction of engine operating conditions.
 10. The method of claim 9,further comprising reducing the target pressure differential at lowerload engine operating conditions compared to higher load engineoperating conditions.
 11. The method of claim 10, further comprisingreducing the target pressure differential at idle compared to higherload engine operating conditions.
 12. The method of claim 9, furthercomprising increasing the target pressure differential as engine loadincreases.
 13. The method of claim 8, further comprising maintaining thetarget pressure differential during transient engine operatingconditions.